1. Field of the Invention
This invention relates to an optical recording medium for recording therein various information with high density by means of laser beam, etc., and reproducing such recorded information from it. More particularly, the invention is concerned with an optical recording medium capable of effecting optical recording and reproduction of information with high density by means of laser beam, etc. having its optical wavelengths in the visible light and near-infrared regions and being of a low energy level.
2. Description of the Prior Art
In general, an optical disc is constructed with a substrate and a thin recording layer provided on the substrate, and has the capability of recording information with high density in the form of optically detectable minute pits of a size of, for example, approximately one micron, which are arranged in the form of spiral or concentric tracks. For writing information into this disc, the surface of the laser sensitive layer thereon is scanned by the laser beam, and the beam-irradiated surface alone has the pits formed therein, the pits being arranged on this optical disc in the form of spiral or concentric tracks.
The laser sensitive layer is capable of absorbing the laser energy to form therein optically detectable pits. For instance, in a heat mode recording system, the laser sensitive layer absorbs heat energy to form minute pits at the irradiated portion due to evaporation or deformation of the material constituting the layer. In another heat mode recording system, the optically detectable pits having a difference in the degree of oxidation caused by chemical changes, difference in the reflection factor, or difference in the optical density can be formed at the irradiated portion due to absorption of the laser energy into the laser sensitive layer.
The information recorded in this optical recording medium is detected by scanning the laser beam along the tracks, and reading optical changes between the portions where the pits have been formed and those where no pits have been formed.
As the recording medium for use in such recording and reproduction, there have so far been known metallic thin films such as aluminum vapor-deposited film, a bismuth thin film, a tellurium oxide thin film, those thin films using inorganic substances such as chalcogenite type amorphous glass film as the principal constituent and those thin films of organic substances such as plastic film containing therein dyes (or coloring matters).
However, in the heretofore known methods of forming the pits by the chemical changes as mentioned in the foregoing, various disadvantages inevitably result. The thus produced recording medium exhibits poor sensitivity of the recording medium low optional contrast between the pitted section and the unpitted section on the recording medium and lack of recorded information having high S/N ratio and instability in storage for the recording medium with the recorded information and other problems. On the other hand, a method of forming holes is able to produce the recording medium with high sensitivity and high S/N ratio, but the method has its own disadvantages such that the surface of the laser sensitive layer exposed to the atmosphere is subjected to oxidation under the environment of moisture-containing or oxidizing atmosphere and becomes deficient in its stability. With a view to removing such disadvantages as mentioned above, there have been proposed conventional methods to provide a protective layer on both upper and lower surfaces of the laser sensitive layer, or either of them. However, such protective layer causes decrease in the sensitivity and the S/N ratio of the recording medium.